This invention relates to the liquefaction of carbonaceous solids such as coal in the presence of a metal-containing hydrogenation catalyst, and is particularly concerned with the recovery of the metal constituents from the residues produced during the liquefaction process and their uses as constituents of the metal-containing catalyst.
Processes for the direct liquefaction of coal and similar carbonaceous solids normally require contacting of the solid feed material with a hydrocarbon solvent and molecular hydrogen at elevated temperature and pressure to break down the complex high molecular weight hydrocarbon starting material into lower molecular weight liquid and gases. Schemes for employing catalysts to promote the liquefaction and hydrogenation of coal in such processes have been disclosed in the prior art. Metals known to be effective catalytic constituents include cobalt, iron, manganese, molybdenum and nickel. These metals may be added directly into the liquefaction zone in the form of water-soluble or oil-soluble compounds, or compounds containing the metals may be directly impregnated onto the carbonaceous feed material. In some cases, the metal-containing compound may be added to the liquefaction zone in the form of a supported catalyst by impregnating the metal-containing compound onto an inert support such as silica or alumina. Since the metals that comprise the catalyst which is eventually formed in the liquefaction zone tend to be expensive, it is necessary to recover the metal constituents for recycle to the liquefaction zone.
Processes have been proposed in the past for separating the metal catalyst constituents from the solid residue of carbonaceous material left after the feed has been converted in the liquefaction zone and the products processed for the recovery of liquids. In one such process it is proposed to pass the liquefaction residue to a synthesis gas generator to produce molten ash containing the catalyst constituents and then treating the molten ash with chlorine or oxygen to convert the metal catalyst constituents to a volatile compound which can be easily recovered. This process is undesirable because of the high temperatures needed to generate the molten ash and volatilize the catalyst constituents. It has also been proposed to recover the metal catalyst constituents by first subjecting the residues from the liquefaction zone to a carbonization step, burning the resultant char and treating the oxidized char from the burning step with a liquid solution of phosphoric or silicic acid to form a heteropoly acid which can then be reused as the catalyst. This technique is disadvantageous because the acid will extract, in addition to the metal catalyst constituents, large amounts of alumina and other metals such as iron from the oxidized char. The alumina and other metals must be separated from the extracted metal catalyst consituents before these constituents can be reused and this adds appreciably to the cost of the process. It is clear that a more efficient method of recovering the metal-containing catalyst constituents is needed.